Plasma arc torches have a wide variety of applications for the working of metals, including cutting, welding, and annealing. Such torches usually include an electrode, also known as a consumable, for supporting an arc that extends from the electrode to a workpiece. A plasma gas such as an oxidizing gas is typically directed through a nozzle assembly to a plasma chamber between the electrode and the nozzle such that the plasma gas is operable to impinge on the workpiece with the gas surrounding the arc in a swirling fashion. The electrode includes an emissive insert which emits electrons upon an electrical potential being applied between the electrode and the workpiece. When the cutting or welding process is complete, the torch may be turned off through a stop program which extinguishes the arc.
It is a common practice to flush oxidizing gas out of the torch following a work operation by flowing a non-oxidizing gas through the passages and nozzle of the torch. The oxidizing and non-oxidizing gases may be selectively introduced into the plasma chamber via a pair of actuatable valves. One or more gas feed lines direct the oxidizing or non-oxidizing gas from the solenoid valves to the plasma chamber at the tip of the torch. Thus, the actuatable valves may be opened and closed to exchange the oxidizing gas for the non-oxidizing gas at the end of the cutting process. A common method for shutting down the torch following a plasma arc cutting process typically includes the following simultaneous operations: switching off the power supply to the electrode; switching off the oxidizing gas, such as oxygen; and switching on a non-oxidizing gas, such as nitrogen, to flush the oxidizing gas from the torch. By simultaneously performing all three operations, however, the plasma arc extinguishes before the non-oxidizing gas completely purges the undesirable oxidizing gas from the plasma chamber of the torch, such that the arc extinguishes in a non-inert environment in the vicinity of the front emitting face of the electrode.
More specifically, the arc extinguishes before the non-oxidizing gas has purged the oxidizing gas from the plasma chamber due to the time delay or lag that is inherent in the purging of the oxidizing gas from the torch. This time delay or lag is due to the volume of gas contained within the tubing and passageways extending between the valves and the plasma chamber adjacent the electrode and nozzle assembly. All of the undesirable oxidizing gas to be purged must be ejected through the nozzle of the torch, which is a time consuming process dependent on the size of the nozzle orifice, the length and volume of the gas tubing, gas passageways and plasma chamber, the rate of flow of the non-oxidizing gas into the tubing, passageways and plasma chamber, and the rate of flow of the oxidizing gas through the nozzle. As a result of the arc extinguishing in a non-inert environment, oxides form on the outer surface of the electrode. Although the formed oxides are typically burned off when the arc is restarted, the repeated formation and burning off of oxides on the electrode contributes to accelerated consumable wear and poor starting performance.
Several methods have been developed to address various deleterious effects that can result when a plasma arc torch is shut down. One method for stopping the cutting operation of the plasma arc torch is described in U.S. Pat. No. 5,166,494, and includes ramping down the mass flow rate of the oxidizing gas as the torch approaches the end of a cutting path. When the electrical current to the torch is cut to extinguish the arc, a residual flow rate of the oxidizing gas remains in the plasma chamber to maintain the stability of the arc, which is said to be desirable to prevent damage to the electrode. As such, the arc extinguishes in a non-inert environment which causes the formation of oxides on the outer surface of the electrode.
Another method for stopping the cutting operation of the plasma arc torch, as exemplified in U.S. Pat. No. 5,070,227, includes venting the plasma chamber to atmosphere while reducing or cutting the flow of oxidizing gas to the plasma chamber, such that the oxidizing gas ejects both through the nozzle of the torch and through the vent to atmosphere to facilitate a more rapid change in the gas flow pattern in the plasma chamber. However, a residual flow rate of the oxidizing gas remains in the plasma chamber to maintain the stability of the arc in order to prevent damage to the electrode. Thus, the arc extinguishes in a non-inert environment which causes the formation of oxides as described above.
Thus, there is a need for a method and apparatus for stopping a cutting operation of a plasma arc torch in which formation of oxidation on the electrode is minimized.